1. Field of Invention
The present invention relates to a multi-color light emitting device circuit; particularly, it relates to a multi-color light emitting device circuit, wherein the number of light emitting devices of each light emitting device string in the multi-color light emitting device circuit is determined by an operational voltage of the light emitting device according to its color.
2. Description of Related Art
A so-called “RGB color sequential technique” is proposed for use in a light emitting diode (LED) projector, in which the red, green and blue LEDs sequentially emit light with a settling time between different colors, such that as a whole the LED projector projects an image with complete colors to a user. For a hand-held LED projector, as shown in FIG. 1, the red, green and blue LEDs typically share one DC-DC power regulator circuit 100 to minimize the size of the projector and reduce the manufacturing cost. In this prior art LED projector, when one color LED (i.e., RLED, GLED, or BLED) is programmed to emit light, a logic gate 12 controls a switch 14 according to a corresponding light emission signal R, G, or B to select a supply voltage Vout to be supplied to a multi-color light emitting device group 20; and in the mean while, a transistor Q1, Q2, or Q3 also turns ON according to the light emission signal R, G, or B, such that a selected color LED string of the multi-color light emitting device group 20 turns ON.
In the prior art shown in FIG. 1, according to the selection by the switch 14, either a voltage drop across a sensing resistor Rs or a voltage at the node between a first resistor R1 and a second resistor R2 is fed back to the DC-DC power regulator circuit 100 so that it generates the proper supply voltage Vout. More specifically, the operational voltages of the red, green and blue LEDs are different. In general, a white LED has an operational voltage of about 3.2V-3.8V; a red LED has an operational voltage of about 1.9V-2.6V; a green LED has an operational voltage of about 2.9V-3.7V; a blue LED has an operational voltage of about 3.0V-3.8V. For simplicity in explaining, in the prior art shown in FIG. 1, the operational voltage of the red LED RLED is assumed to be 2.3V, the operational voltage of the green LED GLED is assumed to be 3.6V, and the operational voltage of the blue LED BLED is assumed to be 3.6V. If the supply voltage Vout is set to be 0V when all the red, green and blue LEDs are OFF (dark status), there will be a large voltage difference (2.3V or 3.6V) in the supply voltage Vout between turning ON one color LED and the dark status, and the circuitry will suffer a slow response time. Therefore, a dark level between the aforementioned operational voltages 2.3V and 3.6V, such as 3V, is provided in the prior art, and when all the red, green and blue LEDs are OFF, the supply voltage is set to this dark level, such that the voltage difference between the dark status and turning ON one color LED ON is reduced, to increases the response speed of the circuitry. In the dark status, all the red, green and blue LEDs are OFF, and the switch 14 switches the DC-DC power regulator circuit 100 to receive a dark feedback signal from a dark feedback circuit 13 (including the first resistor R1 and the second resistor R2) according to the output signal from the logic gate 12. The resistances of the first resistor R1 and the second resistor R2 are properly arranged such that the supply voltage Vout is kept between the aforementioned operational voltages 2.3V and 3.6V, such as 3V.
An example of the waveform of the supply voltage Vout generated by the aforementioned prior art is shown in FIG. 2. Even though the voltage difference between the dark level and turning ON one color LED is reduced, the voltage difference between the operational voltages of the red LED (RLED) and the other two color LEDs (GLED and BLED) is still very large, i.e., 1.3V, or even greater if more LEDs are connected in one LED string. Thus, a relatively long period is required for charging/discharging an output capacitor C1 during the process of switching between the red LED RLED and one of the other two color LEDs (GLED and BLED), such that the switching time between different colors is long and it decreases the image contrast. All in all, the response time of the circuitry is still not satisfactory.
If all the LED strings do not share one DC-DC power regulator circuit, but each LED string has it own DC-DC power regulator circuit, the above issue may be solved; however, this is not cost-effective. Therefore, it is necessary to provide a cost-effective multi-color light emitting device circuit with a relatively simple hardware configuration.
In view of the foregoing, the present invention provides a multi-color light emitting device circuit, in which the number of the light emitting devices of each light emitting device string is determined by the operational voltage of the light emitting device of a color substantially the same as the color of the light emitting devices in that light emitting device string, such that the circuitry response speed is increased while the control circuit has a cost-effective simple hardware configuration.